1. Field of the Invention
The present invention relates to an apparatus for forming a thin film, and more particularly to an apparatus for forming a high-quality thin film by means of an ionized-cluster beam deposition (ICB) method.
2. Description of the Related Art
FIG. 3 is a schematic representation showing a conventional apparatus for forming a thin film disclosed, for example, in Japanese Patent Publication No. 54-9592. The apparatus for forming a thin film has a vacuum chamber 1 to keep its vacuum to a predetermined degree less than 10.sup.-4 Torr. A vacuum exhaust system 2 is connected to the vacuum chamber 1 in order to evacuate the vacuum chamber 1.
A crucible 3 is arranged inside the vacuum chamber 1, this crucible 3 for generating clusters of a substance 5 in the crucible by vaporizing the substance 5. A nozzle 4 is provided over the crucible 3.
Furthermore, the crucible 3 is filled with the substance 5, and heating filaments 6 are arranged surrounding the crucible 3.
Moreover, a heat shielding plate 7 is disposed outside the heating filaments 6 so as to intercept the heat from the heating filaments 6. A vapor source 9 includes the crucible 3, the heating filaments 6 and the heat shielding plate 7.
What is indicated by numeral 8 are clusters (massive atom groups) which are formed by evaporating the substance 5 through the nozzle 4 arranged over the crucible 3.
Ionization filaments 10, which emit electrons for ionization of ions, are arranged over the crucible 3. An electron beam drawing electrode 11 is disposed inside the ionization filaments 10 so as to draw electrons from the ionization filaments 10 and accelerate them.
Furthermore, a heat shielding plate 12 is arranged outside the ionization filaments 10 so as to intercept the heat of the ionization filaments 10. An ionizing means 13 includes the ionization filaments 10, the electron beam drawing electrode 11, and the heat shielding plate 12.
In addition, an acceleration electrode 15a and a ground electrode 15b are arranged over the ionizing means 13. The acceleration electrode 15a and the ground electrode 15b comprise an acceleration means which accelerates, in an electric field, clusters 14 ionized by the ionizing means 13 in order to provide the ionized clusters 14 with kinetic energy. A substrate 16, on which a thin film is deposited, is disposed over the acceleration electrode 15a and the ground electrode 15b.
A first AC power supply 17 is connected to the heating filaments 6 mentioned above. A first DC power supply 18 is also connected to the heating filaments 6, this first DC power supply 18 causing the electric potential of the crucible 3 to be positively biased with respect to the heating filaments 6.
Moreover, a second AC power supply 19 is connected to the above-mentioned ionization filaments 10. A second DC power supply 20 is also connected to the ionization filaments 10, this second DC power supply 20 causing the ionization filaments 10 to be negatively biased with respect to the electron beam drawing electrode 11.
In addition, a third DC power supply 21 is connected to the crucible 3, the electron beam drawing electrode 11, and the acceleration electrode 15a. The third DC power supply 21 causes the crucible 3, the electrodes 11 and 15a to be positively biased with respect to the ground electrode 15b. The first AC power supply 17, the first DC power supply 18, the second AC power supply 19, the second DC power supply 20, and the third DC power supply 21 are all housed in a power supply device 22.
The operation of the apparatus for forming a thin film will be described hereinafter.
The vacuum chamber 1 is evacuated by the vacuum exhaust system 2 to approximately 10.sup.-6 Torr.
Electrons emitted from the heating filaments 6 are drawn out by the electric field applied by the first DC power supply 18. These drawn electrons collide with the crucible 3 to heat it until the vapor pressure in the crucible 3 reaches several Torr.
This heating evaporates the substance 5 in the crucible 3, whereby the substance 5 is injected into the vacuum chamber 1 through the nozzle 4.
The vapor of the substance 5, when passing through the nozzle 4, is accelerated and cooled by means of adiabatic expansion, and is condensed to form the clusters 8.
The second DC power supply 20 causes the ionization filaments 10 heated by the second AC power supply 19 to be negatively biased with respect to the electron beam drawing electrode 11, whereby thermionic electrons emitted from the ionization filaments 10 are introduced into the inside of the electron beam drawing electrode 11.
The clusters 8 then turn into ionized clusters 14 due to ionization by the electron beam emitted from the ionization filaments 10.
The third DC power supply 21 causes the crucible 3, the electron beam drawing electrode 11, and the acceleration electrode 15a to be positively biased with respect to the ground electrode 15b in a ground electric potential. The acceleration of the ionized clusters 14, together with neutral clusters 8 which are not yet ionized, is controlled by means of an electric field lens formed between the acceleration electrode 15a and the ground electrode 15b. The ionized clusters 14 collide, after being accelerated, with the surface of the substrate 16 to form a thin film.
As has been described above, in the conventional apparatus for forming a thin film, the properties of the thin films formed are controlled by providing the ionized clusters 14 and by controlling the kinetic energy of the clusters 14. For this reason, to form homogeneous thin films, it is necessary to lessen the variations in the kinetic energy of the atoms of an ionized cluster beam which collides with the surface of the substrate 16. It is also required that an appropriate quantity of the ionized clusters 14 collide with the substrate 16. This quantity is maintained by altering the acceleration voltage applied by the third DC power supply 21.
When there are variations in the sizes of the clusters, there are also variations in the kinetic energy of the atoms colliding with the surface of the substrate 16.
For example, when a voltage of 600 V is applied to the third DC power supply 21 to accelerate the ionized clusters 14, the ionized clusters 14, each composed of two atoms, collide with the substrate 16, with each atom having an energy of 300 V. At the same voltage, on the other hand, the ionized clusters 14, composed of three, four, and five atoms, collide with the substrate 16, with each atom having an energy of 200 V, 150 V, and 120 V, respectively.
When a single atom which is not formed into a cluster is ionized, it is accelerated with an energy of 600 V.
As mentioned above, there is a problem in that it is impossible to form homogeneous thin films when the kinetic energy of the atoms constituting the clusters which impinge upon the substrate 16 is not uniform.
There is also a problem in that the collisions of small ionized clusters and ionized atoms against the substrate 16 cause damage to the substrate 16 because of the large amount of the kinetic energy at the collision.
As the acceleration voltage varies, so does the amount of ionized clusters drawn. The quantity of such ions is proportional to the 1.5th power of the acceleration voltage, according to the Child-Langmuir equation.
Thus, when the acceleration voltage in particular is made small so as to control the properties of the thin films, the quantity of ionized clusters reaching the substrate 16 greatly diminishes. This results in a problem in that it is impossible to form high-quality thin films by making use of the properties of the ionized clusters.
There is also a problem in that as the acceleration voltage approaches 0, electrons flying out of the ionization filaments 10 impinge upon the substrate 16, thereby causing damage to the substrate 16.